This invention relates to a process for the preparation of 3-amino-4-hydroxybenzoic acids.
Papenfuss, U.S. Pat. No. 3,929,864 discloses a process for the preparation of 4-hydroxy-3-nitrobenzoic acid alkyl esters wherein 4-hydroxybenzoic alkyl esters are contacted with nitric acid at about 0.degree. C. to 60.degree. C. wherein the nitric acid has a strength of 30 to 62 percent by weight so as to prepare 4-hydroxy-3-nitrobenzoic acid alkyl esters. These compounds can be used directly for further reactions, for example, subjected to catalytic hydrogenation to give 4-hydroxy-3-aminobenzoic acid esters.
The Papenfuss process suffers from the drawback of over-nitration and of decarboxylation of the starting material. Purification of the product of the Papenfuss process is difficult and tedious.
3-Amino-4-hydroxybenzoic acids are useful in the preparation of AB polybenzoxazole ordered polymers. Previous methods used to prepare such compounds afford such compounds in low yields requiring extensive purification so that they may be rendered suitable for polymerization. It has been discovered that the presence of impurities in 3-amino-4-hydroxybenzoic acids prevent the formation of high molecular weight AB polybenzoxazoles. What is needed is a process which prepares 3-amino-4-hydroxybenzoic acids in high yields with high purity.